• Preheat the oven to 375°F, and place a baking sheet on the lowest rack, to catch any drips when the cake bakes. Grease and line with parchment a 9-inch square pan and set aside.
• Put the sugar, unsweetened chocolate, butter and 1 cup of water in a saucepan. Place over medium heat and stir occasionally until all of the ingredients are melted and blended. Remove from the heat and let cool slightly.
• Meanwhile, sift together the flour, baking soda, baking powder and salt.
• In a small bowl, stir together the milk and vinegar.
• When the chocolate in the pan has cooled a bit, whisk in the milk mixture and eggs. In several additions and without over mixing, whisk in the dry ingredients. When the mixture is smooth, add the vanilla and whisk once or twice, to blend. Pour the batter into the pan and bake on the middle rack until a skewer inserted in the center comes out clean, about 30 to 35 minutes.
• Let the cake cool completely, then remove from the pan and cool on a rack.

  Chocolate Cake recipe from Amamda Hesser – Food52

For the Frosting

• Combine room temperature butter, cocoa, vanilla and confectioners’ sugar with a hand mixer until well combined.
• Slowly add milk, one tablespoon at a time, until you reach the desired consistency.
• Spread frosting on cooled cake.